Over recent years, there has been increasing concern over the consumption of fossil fuels and the production of greenhouse gases. One way to reduce the globe's reliance on fossil fuels has been the development of biofuels from renewable sources. Biofuels such as biodiesel and bioethanol are considered to be cleaner and more environmentally friendly alternatives to fossil fuels.
Although biofuels may help in reducing green house emissions, they are not without problems. A controversial aspect is the “food for fuel” problem where the demand for energy crops has been perceived as pushing up the prices of grain commodities. Another serious drawback is the damage caused to ecologically sensitive ecosystems, such as rain forests, where the planting of energy crops such as soya and palm has caused large scale destruction.
The biofuels industry is turning to second and third generation biofuels to alleviate these issues. The production of fuels by micro-organisms (1) and the use of waste substrates (2) are important areas of research.
The conversion of carbon dioxide to fuel molecules is known. Carbon dioxide can be converted chemically (3), electrochemically (4), and either directly (5) or indirectly (6) by micro-organisms. Products such as formic acid, formate, methanol, formaldehyde, ethylene, methane and oxalic acid have been noted. However, these micro-organisms cannot convert carbon dioxide via formic acid into a longer chain energy source such as aliphatic carboxylic acids.
In US 2012003705, the conversion of carbon dioxide to biomass is described (7) and then the further processing of the biomass to a range of commercially useful molecules. However, this is not done through the steps of fixing carbon dioxide to formic acid and then converting the formic acid to aliphatic carboxylic acids.
Previous attempts at using carbon dioxide as a carbon substrate to produce fuel molecules have had limitations. Carbon dioxide and its aqueous ions bicarbonate and carbonate are inherently stable and the Gibbs free energy of formation are the most electronegative of the carbon molecules. To convert carbon dioxide to fuel molecules requires a large input of energy (heat), extreme conditions (pressure) and highly reactive chemicals (catalysts). Yields are often poor and the rates of reaction slow. Chemical approaches to the direct use of carbon dioxide are generally not considered to be economically viable. Likewise, the initial production of biomass by chemolithotrophic bacteria is not widely practised due to cost constraints of growth and downstream processing.
Electro-catalysis has also had limited success. Outcomes have been limited by the poor solubility of carbon dioxide in water (0.033M) and the energetic requirements of a reaction with a strong electronegative potential (E0=−0.61V). Electro-catalysis is also a costly technology requiring high quality metals for electrode surfaces. Production of longer chain products have been described in terms of Fischer Tropsch type reactions (8), but again chains are limited in length. Photoreduction on irradiated semi-conductor surfaces produces carbon monoxide, formate, methanol, methane, formaldehyde, oxalic acid and glyoxal. Again this is a costly technology with low yields.
While enzymes such as bacterial formate dehydrogenase are known to reduce carbon dioxide to formate (9), the forward reaction (oxidation of formate to carbon dioxide) is generally favoured because NADPH is required to drive the reaction and the reduction potential of NADP is more positive than that of carbon dioxide. Such a reaction also requires electron donor and acceptor molecules. Tungsten containing enzymes from Syntrophotobacterium fumioxidans (10) are able to carry out this reaction but require absorption onto an electrode surface for the electro-catalytic system to function efficiently. Further, longer fuel molecules are not produced.
The present invention is an improvement compared to the prior art in that carbon dioxide can be converted to an initial platform molecule (formic acid) and then assembled into longer chains in a rapid format that does not require fermentation or the generation and processing of biomass. This improves on known methods such as US2012/0003705 (11), which requires the production of biomass and recycling of electron donors and acceptors, and US 2010/03170741A1 (12), US 2012/0003706A1 (13), US2012/003707A (14) and US2012/0034664A1 (15) which all require fermentative processes.